Hydropneumatic accumulator

ABSTRACT

Hydraulic accumulator comprising an accumulator housing ( 1 ) and a diaphragm ( 3 ) located therein, which forms a displaceable partition element between a first chamber, in particular a gas chamber ( 7 ) and a second chamber, in particular a liquid chamber ( 5 ). Said membrane has several annular regions ( 31, 33, 35, 37, 39 ), interconnected by annular weak points ( 61, 63, 65, 67, 69, 71 ). The annular regions ( 31, 33, 35, 37, 39 ) have protuberances ( 81, 83, 85, 87, 89 ) on the interior face of the membrane ( 3 ) facing away from the wall of the accumulator housing ( 1 ). Said protuberances are at their thickest in the central region between neighboring weak points, thus increasing the wall thickness of the membrane ( 3 ) and are at least partially convex ( 91, 93 ), tapering off to a respective flat form towards the weak points.

[0001] The invention relates to a hydropneumatic accumulator with the features of the preamble of claim 1.

[0002] The pertinent hydropneumatic accumulators are known in a host of constructions and embodiments, one of the main tasks of hydropneumatic accumulators (accumulators) being to accommodate certain volumes of pressurized liquid of a hydraulic system and to return it if necessary to the system.

[0003] DE-A-41 31 790 discloses a bladder or membrane for the pertinent accumulator which comprises an elastic material layer, with a gas blocking layer which has been laminated onto it and with an elastic fastener which is made on the inside surface of a peripheral edge piece of the elastic material layer so that it or the material layer can be pressed onto the housing if the fastener is attached to the housing by means of a holding element. The membrane is made W-shaped in cross section, the thickness of the elastic material layer decreasing in an interposed section of the latter and/or it's increasing on the curved reverse area of the material layer. The convex middle area of the W-shaped membrane is therefore reversed to the edge of the membrane and consequently in the direction of the fastener into a concavely running curvature and in spite of the thickness increase of the membrane intended in this area for a plurality of load cycles, its edge tearing and consequently failure of the accumulator cannot be precluded.

[0004] Conversely, in a generic accumulator as claimed in DE-A-40 18 318 it has already been suggested that the membrane be divided into zones which are articulated to one another in order in this way to help prevent uncontrolled unfolding of the membrane during its working movements and in this way to achieve longer service lives. In the known approach, conversely, between the deflection points of the membrane in the form of film hinges the membrane is made embossed in plate or strip form; but this can be unfavorable in the process of unwinding of the membrane, especially when the plate-shaped elevations abut one another and thus increased force application in the area of the film articulations occurs.

[0005] DE-A-1 675 349 discloses a generic hydropneumatic accumulator with an accumulator housing and a membrane which is located therein and which forms a movable separating element between a first chamber, especially a gas chamber, and a second chamber, especially a liquid chamber, which has several annular areas which are interconnected by way of annular weak points which act like articulations, the annular areas having on the inner side of the membrane which faces away from the wall of the accumulator housing elevations which in the central area between adjacent weak points have the greatest height which increases the wall thickness of the membrane and a shape which tapers off towards these weak points and which is convex at least in partial areas. In the known approach the elevations form annular beads or ribs which project out of the plane of the separating element in the direction of the inner side of the membrane and which with their convex arches taper off steeply in both directions and pass in this way into elongated annular wall areas of the separating membrane which, viewed in the lengthwise direction of the hydraulic accumulator, keep the elevations in the initial position of the separating element at equal distances to one another. Due to the sharply delineated transitions between the bead-shaped or annular elevations and the weak points which are formed from parts of the separating element with the same wall thickness, kinks in the membrane are formed, with the result that when it moves overstresses can form locally and accordingly it can fail.

[0006] On the basis of this state of the art, the object of the invention is to devise an accumulator of the type under consideration which is characterized by especially good operating behavior of the membrane so that a long service life can be achieved even during operation with high pressure ratios and high rates of pressure change.

[0007] In an accumulator of the initially mentioned type, this object is achieved as claimed in the invention in that the elevations at least in the area bordering the adjacent weak points taper off to the tangential plane of the pertinent weak points, which plane is tangential relative to the stretched shape of the membrane, to the weak points, at a flat angle which is less than 20°.

[0008] The shape intended as claimed in the invention with the areas of the membrane which taper off flatly towards the articulations avoids the danger that when the membrane folds, overly sharp kinks at the joints form as is the case in the aforementioned, known generic accumulators. Thus, the damaging notch effect is prevented. Conversely the articulated movements for the shape as claimed in the invention take place with a certain radius of curvature, by which the danger of local overstress of the membrane is prevented. Without overly enlarging the wall thickness of the membrane thus comparatively large pressure amplitudes and rates of pressure change are allowable.

[0009] DE-A-28 52 912 for an accumulator with a thick-walled separating element of rubber discloses the formation of inner annular areas on the separating element with bead-like elevations. But they are beads or ribs which project out of the plane of the separating element without the beads or ribs having to be separated from one another by weak points which act in the manner of a film hinge. Moreover the beads in the areas adjacent to one another have a steeply tapering shape so that a careful unwinding process cannot be achieved with this known membrane.

[0010] In one preferred embodiment of the accumulator as claimed in the invention, the weak points have a concave shape, in the membrane there being an alternating sequence of convex with concave areas. This yields especially good bending and unrolling properties in the folding of the membrane. Preferably the membrane in its initial position along its outer surface has a permanent and essentially uniform curvature and the membrane is designed in the manner of a hemisphere in its initial position.

[0011] Preferably the angle between the area of the elevations which adjoins the bordering weak points and the tangential plane which belongs to the pertinent weak points, relative to the stretched shape of the membrane, is less than or equal to 15°.

[0012] Especially good bending properties arise when the membrane forms folds if the elevations in the area bordering the weak points have a concave arch and in the area adjacent to the greatest height of the elevation, a convex arch. At least parts of the elevations however can also be formed by planar surfaces which preferably taper off towards the adjacent weak points.

[0013] In a membrane with a circular cross section and with annular areas which are arranged concentrically to one another and which are bordered by weak points which form circles, the annular areas or at least some of them can be divided by weak points which run transversely to the circles, these weak points preferably running radially relative to the circle cross section.

[0014] The invention makes it advantageously possible to produce the membrane from a gas-tight monolayer plastic, for example from a polyamide, a polyamide blend, polyethylene terephthalate, polyethylene naphthalate or polyvinylidene chloride.

[0015] The invention is detailed below using embodiments shown in the drawings.

[0016] The drawings are schematic and not to scale.

[0017]FIG. 1 shows a lengthwise section through an accumulator according to one embodiment of the invention;

[0018]FIG. 2 shows a representation of the membrane of the accumulator from FIG. 1, which membrane is in its stretched state, and which representation is on a larger scale, on one side and is highly schematically simplified;

[0019]FIG. 3 shows a top view of the membrane of the embodiment which is drawn on the scale corresponding to FIG. 1, and

[0020]FIG. 4 shows a top view of the membrane of a modified embodiment similar to that of FIG. 3.

[0021] The accumulator shown in FIG. 1 is a so-called membrane accumulator with a separating element which is located in the housing 1, in the form of a membrane 3. The membrane 3 separates the housing 1 in its interior into a first chamber, in this example a liquid chamber 5, and into a second chamber, in this example a gas chamber 7. Both the liquid chamber 5 and also the gas chamber 7 each have a connection 9 and 11 by means of which the accumulator can be connected to the pipelines of a hydraulic system (not shown) and by way of which the gas pressure can be set. Comparable accumulators with the previously described features are state of the art, so that the accumulator is explained below only to the extent which is necessary for description of the invention.

[0022] The housing 1 consists, when viewed in the line of sight to FIG. 1, of a top housing shell 13 and a bottom housing shell 15 which abut one another along a seam 17. Along this seam 17 the two shells 13 and 15 can be joined to one another by means of an electron beam welding process or laser welding to form the housing 1.

[0023] Within the housing 1, to hold the membrane 3 which is shown in FIGS. 1 and 2 in its stretched state, there is a steel ring 19 with a height such that it covers the seam 17 within the housing 1 with an excess length. On its side facing the gas chamber 7 the ring 19 has recesses in the form of notches 21 which are made rectangular in cross section and which are routed around the ring 19 at the same distances from one another. On its side facing the liquid chamber 5 the ring 19 on its side facing the inside wall of the housing has a peripheral annular groove 23 with a lower edge, viewed in the direction of looking at FIG. 1, which forms a shoulder surface 25 as an abutment for the edge bead 27 of the membrane 3 which has a rectangular cross section. The cross section of the edge bead 27 and the depth of the annular groove 23 are such that the membrane on the edge bead 27 is held gas-tight on the inside wall of the housing if the ring 19 is fixed in the accumulator housing. This takes place in the course of the welding process which is carried out after seating the upper housing shell 13, in order to weld the housing 1 on the seam 17, the ring 19 being welded tightly to the housing 1.

[0024] Details of the configuration of the membrane 3 can be taken from FIGS. 1 to 3. FIGS. 1 to 3 show that the membrane 3 in an overhead view for a round cross section has a shape which is entire outer surface, aside from the end-side edge bead 27, has a smooth surface, but on its inside it has contouring. In the embodiment shown in FIGS. 1 to 3 the contouring is such that the membrane 3 on its inside is divided into concentric annular areas 31, 33, 35, 37 and 39. These annular areas are bordered laterally each by weak points 61, 63, 65, 67, 69 and 71 in which the membrane 3 has the smallest wall thickness. These weak points extend along the periphery of the inside of the membrane along circles 41, 43, 45, 47, 49 and 51. In the annular areas 31, 33, 35, 37 and 39 between the weak points there are flat, bead-like elevations 81, 83, 85, 87 and 89, see FIG. 2.

[0025] As is clear from FIG. 2, the elevations 81, 83, 85, 87 and 89 each have their greatest height which corresponds to the greatest wall thickness of the membrane in the central area between the bordering weak points 61, 63, 65, 67, 69 and 71. In this example, the elevations in this central area each have a convex arch 91. One concave area 93 which extends up to the bordering weak points laterally adjoins the convex arch 91. The concave areas 93 extend tapering off flatly to the respectively assigned weak points, the flat angle of incline relative to the tangential plane of the membrane 3 which is in the stretched state, which plane is tangential relative to the respective weak point, being preferably on the order of 12° to 14°.

[0026] Based on this configuration with elevations 81, 83, 85, 87, 89 which rise gently at the weak points 61, 63, 65, 67, 69, 71, the weak points, when folds form in the membrane 3 as a result of the working motion of the membrane 3, form articulations in the manner of film hinges on which folding of the membrane takes place in a controlled manner, but as a result of the configuration as claimed in the invention no sharp kinks are formed, so that local overstress of the membrane material is prevented and high operating service life is achieved.

[0027]FIG. 4 shows a second embodiment in which the membrane 3 in addition to the weak points which run along the circles 41, 43, 45, 47, 49, 51 are made articulated by radially (relative to the circular shape) running weak points 95. These radially running weak points 95 divide the bead-like elevations which are located within the annular areas and which have the greatest height in the central area between the bordering weak points in exactly the same manner as is shown in FIG. 2 for the first embodiment. From these areas of greatest prominence the elevations taper off in the corresponding manner as in the example from FIG. 2 to the bordering weak points which they approach at a flat angle. Here there can be a concavely/convexly curved arch in the corresponding manner, as in FIG. 2, the concave arched areas approaching both the weak points along the circles 41, 43, 45, 47, 49, 51 and also the radially running weak points 95.

[0028] Instead of the arched shape shown in the figures, there can be other arched shapes or combined shapes composed of arched areas and plane surface areas, however the approach to the respectively bordering weak points taking place at flat angles. Nor is it necessary that, in the embodiments in which according to FIG. 4 there are additional weak points which run transversely to the circles, they are provided in the number as shown in FIG. 4. But these transversely running weak points need not extend beyond all the annular areas and can run other than in a star-shape or radially. The film hinges shown in FIGS. 3 and 4 as weak points are shown for the sake of simplicity as concentric circles with the same distance to one another. As is shown in FIG. 2, the distances however are different, especially increasing from the outside to the inside toward the membrane center.

[0029] The material for the membrane 3 is preferably gastight monolayer plastics, for example polyamide, such as PA6, a polyamide blend, for example PA polyolefin, or polyethylene terephthalate or polyethylene naphthalate or polyvinylidene chloride. But it goes without saying that also other types of materials can be provided for producing the membrane. The radius of curvature for the concave weak points is smaller than the radius of curvature for the convex elevations; in this respect they are less strongly curved than the adjacent weak points. 

1. A hydropneumatic accumulator with an accumulator housing (1) and a membrane (3) which is located therein and which forms a movable separating element between a first chamber, especially a gas chamber (7), and a second chamber, especially a liquid chamber (5), which has several annular areas (31, 33, 35, 37, 39) which are interconnected by way of annular weak points (61, 63, 65, 67, 69, 71) which act like articulations, the annular areas (31, 33, 35, 37, 39) having on the inner side of the membrane (3) which faces away from the wall of the accumulator housing (1) elevations (81, 83, 85, 87, 89), which (81, 83, 85, 87, 89) in the central area between adjacent weak points have the greatest height which increases the wall thickness of the membrane (3) and a shape (91, 93) which tapers off flatly towards these weak points and which is convex at least in partial areas, characterized in that the elevations (81, 83, 85, 87, 89) at least in the area bordering the adjacent weak points (61, 63, 65, 67, 69, 71) taper off at a flat angle to the tangential plane of the pertinent weak points (61, 63, 65, 67, 69, 71), which plane is tangential relative to the stretched shape of the membrane (3), to these weak points, which angle is less than 20°.
 2. The accumulator as claimed in claim 1, wherein the weak points (61, 63, 65, 67, 69, 71) have a concave shape and wherein in the membrane (3) there is an alternating sequence of convex with concave areas.
 3. The accumulator as claimed in claim 1 or 2, wherein the membrane (3) in its initial position along its outer surface has a permanent and essentially uniform curvature.
 4. The accumulator as claimed in claim 3, wherein the membrane (3) in its initial position is designed in the manner of a hemisphere.
 5. The accumulator as claimed in one of claims 1 to 4, wherein the flat angle is ≦15°.
 6. The accumulator as claimed in one of claims 1 to 5, wherein the membrane (3) has a circular cross section and the annular areas (31, 33, 35, 37, 39) are arranged concentrically to one another and are bordered by weak points (61, 63, 65, 67, 69, 71) which form circles and wherein at least some of the annular areas (31, 33, 35, 37, 39) can be divided by weak points (95) which run transversely to these circles.
 7. The accumulator as claimed in one of claims 1 to 6, wherein the membrane (3) is formed from a gastight monolayer plastic.
 8. The accumulator as claimed in claim 7, wherein the material for the membrane (3) is a polyamide, such as PA6, a polyamide blend, for example PA polyolefin, or polyethylene terephthalate or polyethylene naphthalate or polyvinylidene chloride. 